Bryum knowltonii Barnes, Bot. Gaz. 14: 44. 1889 (as knowltoni)

Plants in dense turfs, green or yellow-green. Stems 0.5-2(-3) cm, evenly foliate to comose, innovations elongate and evenly foliate; weakly radiculose. Leaves green, somewhat imbricate to weakly contorted or shrunken when dry, ovate, strongly concave, 1-2(-3) mm, somewhat enlarged toward stem apex; base not decurrent; margins revolute to mid leaf, limbidium strong, in (2-)3-4(-5) rows; apex acute to obtuse; costa percurrent to short-excurrent, awn , denticulate; proximal laminal cells 3-4:1, same width as more distal cells; medial and distal cells hexagonal, 14- 18 µm wide, (2-)3-4:1, walls thin. Specialized asexual reproduction absent. Sexual condition synoicous. Seta (2-)3-4 cm. Capsule yellow-brown, obovate to short-pyriform, symmetric, 2-3 mm, mouth red; operculum short-convex, apiculate; peristome reduced; exostome teeth yellow to orange proximally, hyaline distally, lamellae straight mid tooth, pores absent along mid line; endostome not adherent to exostome, basal membrane high, less than 1/2 exostome height, segments with ovate perforations, cilia absent or rudimentary. Spores (18-) 20-30 µm, finely papillose, pale yellow or green.

Capsules mature Jul-Aug. Wet soil in arctic-alpine; low to high elevations (0-4000 m); Greenland; Alta., B.C., Nfld. and Labr., N.W.T., Nunavut, Yukon; Alaska, Colo., Wyo.; Eurasia.

Ptychostomum knowltonii is a distinctive species of small, evenly foliate plants with concave, ovate, keeled leaves, and a short capsule on a long, slender seta. The costae are red and prominent abaxially. In some collections, the capsules are of distinctly different ages in the same tufts.